1. Field of the Invention
This invention relates generally to oscillators. More particularly, this invention relates to a circuit for linearizing a high frequency voltage controlled oscillator.
2. Description of the Prior Art
Voltage controlled oscillators are today commonly used to generate frequency sweep signals wherein the output signal frequency is continuously varied over a selected frequency range as a function of time. Fir mnost applications it is important to have the output frequency signals change at a constant linear rate.
Heretofore, substantial efforts have been made in developing control circuits for linearizing voltage controlled oscillators. Most such control circuits of the prior art are well known to the artisan; hence, a discussion thereof is unnecessary at this time.
However, there are several prior art control circuits which are of some significance, inasmuch as they indirectly concern subject matter that is pertinent to the linearizing circuit for a high frequency voltage controlled oscillator constituting the instant invention.
For example, U.S. Pat. No. 4,038,612 to R. P. Borofka and L. R. Barton discloses a circuit which makes use of a controllable oscillator (VCO) responsive to a control voltage ramp. Accurately controlled error-sensing means are provided for detecting variations of the instantaneous frequency of a frequency ramp generated by the VCO at predetermined accurately-timed points. The discrete errors at these plural points throughout the frequency ramp are computed and integrated over a plurality of sweep cycles and then are used to compensate the control voltage ramp.
U.S. Pat. No. 4,129,832 to G. W. Neal and R. M. Montgomery dicloses a circuit for linearizing a voltage controlled oscillator which has a random access digital controlled memory coupled with the voltage control terminal of a voltage controlled oscillator to be linearized, a power divider coupled with the output terminal of the oscillator, a mixer coupled with the power divider by two conductive lines of differing time delays, and an X-Y axis signal display coupled with the mixer, whereby the memory may be programmed to input a ramp control signal to the oscillator and a difference frequency signal detected in the chirp output signal generated by the oscillator and compared with a frequency signal on the display to detect nonlinearities in the chirp output signal of the voltage controlled oscillator which may be substantially reduced by programming of the control memory.
Although the aforementioned circuits of the prior art have linearized voltage controlled oscillators, their degree of success has been limited. Typically, utilization of these prior art devices have achieved linearization in the range of .+-.0.5% with the upper limit being approximately .+-.0.1%. In addition, the time involved in performing a linearization operation on individual oscillators has been quite substantial. Furthermore, the aforementioned devices of the prior art do not operate in exactly the same manner as the subject invention and contain a combination of elements that is somewhat different from the present invention.